In current mobile networks, a wireless base station which is statically installed on the ground performs communication with mobile wireless terminals typified by cellular phones or a smartphones. On the other hand, it has been examined that a mobile network includes a wireless base station installed on a moving object such as a train or a bus, wherein the wireless base station (hereinafter, referred to as an “on-vehicle station”) integrates communication of mobile wireless terminals, and the on-vehicle station performs communication with a wireless base station (hereinafter, referred to as a “ground station”) installed on the ground (see, for example, Non-Patent Literature 1). Generally, a moving object such as a train or a bus having an on-vehicle station mounted thereon moves at higher speed than pedestrians. For this reason, hereinafter, a moving object such as a train or a bus is defined as a high-speed moving object and is thus distinguished from a low-speed moving object such as a pedestrian.
FIG. 7 is a schematic diagram illustrating a specific example of a mobile network in the related art. FIG. 7 shows a mobile network in which communication between communication devices 11-1 to 11-3 belonging to a higher-level network 10 and an on-vehicle station 1 is relayed by ground stations 2-1 to 2-3 and a higher-level device 3. FIG. 7 shows a state in which communication of the on-vehicle station 1 during its movement in a traveling direction is relayed to the higher-level network 10 side by the ground station 2-1 and the higher-level device 3. The on-vehicle station 1 belongs to a wireless communication range 21 (hereinafter, referred to as a “cell”) which is managed by the ground station 2-1. Cells 21, 22 and 23 covering the course of the on-vehicle station 1 are adjacently present in the traveling direction of the on-vehicle station 1. The on-vehicle station 1 can maintain communication with the higher-level network 10 side by switching cells to which it belongs with its movement in the traveling direction. Hereinafter, in a case where the ground stations 2-1 to 2-3 are not required to be particularly distinguished from each other, the ground stations 2-1 to 2-3 are generally described as the ground station 2.
The number of optical fibers, which are required to be constructed for connecting each ground station 2 to the higher-level device 3, must be the same as the number of ground stations 2 installed. For this reason, high costs are incurred in order to form cells such as those in which all the courses of the on-vehicle station 1 are covered. In addition, the transmission bandwidth of an optical fiber between each ground station 2 and the higher-level device 3 is used only when the on-vehicle station 1 is present in a corresponding cell of each ground station 2. As shown in FIG. 7, since the on-vehicle station 1 moves while switching cells to which it belongs, the bandwidth utilization efficiency of optical fibers is not high, and an improvement in cost effectiveness is required.
Regarding such a problem, a technique is proposed to improve the bandwidth utilization efficiency of an optical network connecting each of the ground stations 2 and the higher-level device 3 to each other by sharing the optical network among the ground stations 2 and to reduce the costs for laying the optical fibers. As a representative method, a method has been proposed for realizing a mobile network using a PON system obtained by extending a time division multiplexer-passive optical network (TDM-PON) and a wavelength division multiplexing-passive optical network (WDM-PON), which are widely introduced for mass users, or a PON system obtained by integrating a TDM-PON and a WDM-PON (see, for example, Patent Literature 1 and Non-Patent Literature 2 and 3).
FIG. 8 is a schematic diagram illustrating a specific example of a mobile network constituted by an extended PON system. FIG. 8 shows a mobile network in which an optical transmission line between each ground station 2 and a higher-level device 3 is realized in a PON system. The PON system shown in FIG. 8 includes optical network units (ONUs) (optical-terminating devices) 7-1 to 7-3 that connect ground stations 2-1 to 2-3 to the PON system, an optical coupler 8 that integrates the ONUs 7-1 to 7-3, and an optical line terminal (OLT) (optical terminal station device) 9 that connects the higher-level device 3 to the PON system. The OLT 9 communicates with the ONUs 7-1 to 7-3 through the optical coupler 8. In the mobile network configured in this manner, a portion of an optical fiber between the ground stations 2 and the higher-level device 3 can be shared by a plurality of cells. Hereinafter, in a case where the ONUs 7-1 to 7-3 are not required to be particularly distinguished from each other, the ONUs 7-1 to 7-3 are generally described as the ONU 7.